1. Field of Invention
The present invention relates to a supported noble metal catalyst useful in organic and organosilicon processes. More particularly, the present invention relates to a catalyst comprising at least one noble metal deposited on a low surface area base metal having utility in processes comprising carbon-hydrogen or carbon-silicon bond formation, especially hydrosilation processes comprising an Si-H reactant.
2. Prior Art
Catalysts are generally classified according to the phase relationship between the catalyst and the initial reactants. A heterogeneous catalyst is in a different phase (gaseous, liquid, or solid) as compared to the phase of the initial reactants. A homogeneous catalyst is in the same phase as the reactants.
Noble metal catalysts can be either heterogeneous or homogeneous. Further, heterogeneous noble metal catalysts can be unsupported or supported on a carrier comprised of an inert solid material such as a metal oxide, as illustrated by alumina, or a base metal.
In general heterogeneous noble metal catalysts, supported and unsupported, have the advantage of being easily removed from a reaction, such as, for example, by filtration. Such facile removal of the heterogeneous noble metal catalyst can further result in the catalyst being re-used in the same reaction or recovered and used in a different reaction. Such advantages increase the overall efficiency of the catalyst and/or provide cost savings. Heterogeneous noble metal catalysts can be physically attached or fixed in different locations in the equipment in which the reaction is conducted. Heterogeneous noble metal catalysts, including metal oxide supported catalysts, are also readily susceptible to chemical promotion or activity modifications. However, such catalysts generally have the disadvantages of having large agglomerates of metal, and, hence, a much lower level of catalytic activity is observed as compared to homogeneous noble metal catalysts.
Although homogeneous noble metal catalysts usually have the advantage of being more active than heterogeneous noble metal catalysts, a homogeneous noble metal catalyst is usually in the form of a solution and is by definition interspersed among the initial reactants, making separation disadvantageously difficult.
Homogeneous noble metal catalysts must actually be considered as two distinct groups: those that are truly homogeneous, consisting of single atoms or ions of noble metals, and those consisting of agglomerates, frequently called clusters or colloids, of noble metals. The colloids are in solution and cannot be easily separated, as by filtration, and are commonly referred to as colloidal homogeneous noble metal catalysts. In most organosilicon processes, and many organic processes, the homogeneous noble metal catalyst is colloidal.
Means of differentiating these three types of noble metal catalysts, heterogeneous, homogeneous, and colloidal homogeneous, are discussed by L. N. Lewis and N. Lewis in Journal of the American Chemical Society, Volume 108, page 7228 (1986}. Both a heterogeneous noble metal catalyst and a truly homogeneous noble metal catalyst are unaffected by mercury, while a colloidal noble metal catalyst is poisoned, i.e. no catalytic activity remains.
Surprisingly, the process of the present invention produces a catalyst having activity comparable to a colloidal homogeneous noble metal catalyst but, like a heterogeneous noble metal catalyst, it can be readily removed, reused, or recovered from the reaction zone and is susceptible to activity modifications. The catalyst of the present invention is used in organic processes such as hydrosilation, dehydrocondensation, deamination, amination, hydrogenation, dehydrogenation, and the like. That is, the catalyst of the present invention can be substituted for a colloidal homogeneous noble metal catalyst that may be presently used in these processes. In many of these processes, the colloidal homogeneous noble metal catalyst is non-recoverable. The catalyst prepared in accordance with the process of the present invention is recoverable and reusable which can be a significant processing advantage in these processes.